Plate Tectonics

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Transcript of Plate Tectonics

Plate Tectonics Alfred Wegener 1910, theory of continental drift, first published in 1915 the continents had been joined together at one time but had drifted apart evidence South America & Africa looked like they fit together like a puzzle fossils of plant and animal life matched on the 2 continents rock layers matched on the 2 continents problem Wegener could not explain HOW the continents moved theory was mostly forgotten 1960's Harry Hess, geologist claimed "sea-floor spreading" is what caused continents to move evidence mid-ocean ridge chain of volcanic mountains in the ocean rift-valley: deep valley between mountains in the chain location of sea-floor spreading slow, regular process no explosive, volcanic actions that build land sea-floor spreading magma rises from the mantle cools and forms new crust, forming small amounts of new ocean floor fills gap at ridge in rift-valley newly formed crust moves slowly away from ridge like a giant conveyor belt with only sea-floor spreading, earth should "grow", but it doesn't subduction sea-floor spreading is part of a recycling process new crust is created at mid-ocean ridges old crust is pushed back into deep sea trenches old crust melts into mantle; keeps earth from "growing" evidence rocks closest to ridge are younger than rocks found farther away magnetic reversals were identical on both sides of ridges theory of continental drift was combined with theory of sea-floor spreading to form the theory of plate tectonics lithosphere broken up into tectonic plates a thin layer of crust on top of a layer of cool, hard rocks can be continental or oceanic most plates have both continental and oceanic 12 large plates, and a few smaller plates all plates combined make up the earth's surface asthenosphere underneath lithosphere tectonic plates "float" on top of asthenosphere partly molten, flowing layer of rock when one plate moves, all others are affected some plate movement results in sea-floor spreading Atlantic Ocean is spreading or growing North America & Europe are moving further apart at about 1 - 2 centimeters/year There are 3 types of plate boundaries divergent boundary two plates move away from each other example: mid-ocean ridge also called "contructive" boundaries new ocean floor is being "constructed" at these boundaries convergent boundary plates come together under enough pressure, one plate is pushed under another also called "destructive" boundaries subduction happens at these boundaries plate material is destroyed at the subduction zones the lithosphere is melted into the mantle transform boundary also called transform faults faults are deep cracks in the earth's surface plates slide past each other example: one plate moves north, while the adjacent plate moves south crust is neither produced nor destroyed at these boundaries Pacific Plate largest plate edges make up the "Ring of Fire" a line of volcanoes that are found along major trenches as plates collide along convergent boundaries, friction and pressure builds earthquakes occur often, sometimes being severe as plate material is subducted, some of the magma is forced upwards and produces volcanoes San Andreas Fault zone in California famous transform fault on land approximately 1,300 km long and more than 10 km wide in some places runs through 2/3 of the state of California Pacific Plate grinds past the North American plate move about 5 centimeters/year frequent earthquakes earth undergoes constant change formation of mountains, volcanoes, etc. are all explained by the theory of plate tectonics